Fix Remarkup styling in Maniphest and Differential

Summary:
This makes some of the line spacing, paragraph spacing and layout
less terrible. In particular, fixes code blocks inside Differential inline
comments.

Test Plan:
Looked at Maniphest Tasks, Differential Revisions and Differential
inline comments with various flavors of remarkup in them.

Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason
CC: jungejason
Differential Revision: 89